1. Field of the Invention
The present invention relates to a tuner for a cable modem, used for a cable modem enabling high speed data communication at home utilizing a channel, which is not used, of a cable television.
2. Description of the Background Art
For cable television (hereinafter simply referred to as CATV), a so-called HFC (Hybrid Fiber/Coax) has been introduced in which main network is implemented by optical fibers while coaxial cable is still used as incoming line to the distributor. This introduction is to provide broad band data communication service of several M bits/sec. to ordinary homes. It is possible to provide a high speed data line having transmission speed of 30 Mbits/sec. with the band width of 6 MHz simply by using 64 QAM (Quadrature Amplitude Modulation), which is no longer the state of the art. Cable modem is used for such application. Utilizing a channel not used of CATV, high speed data communication of 4 Mbits/sec. to 27 Mbits/sec. is possible.
FIG. 18 is a block diagram of a conventional example of a tuner for a cable modem. CATV signals include an upstream signal transmitted to a station having frequency of 5 MHz to 42 MHz and a downstream signal transmitted from the station to a tuner for a cable modem having the frequency of 54 MHz to 860 MHz, and the signals are connected to a cable circuit through an input terminal 11 of the tuner. The upstream signal transmitted from the cable modem is received by a data receiver of the CATV station (system operator) and enters a computer of the station. A data signal which has been subjected to quadrature phase shift keying (QPSK) from a QPSK transmitter is input to a data terminal 10 of the cable modem as an upstream signal. The data signal is past to an upstream circuit 9 and a CATV input terminal 11 to be transmitted to the CATV station.
A data signal received at the CATV station is subjected to 64 QAM modulation and transmitted to a cable circuit, and enters through a CATV input terminal 11 to the cable modem, as a downstream signal. In the modem, a desired signal is selected by a tuner, the signal is subjected to 64 QAM demodulation, MPEG reproduction and processing at a CPU (Central Processing Unit), and the resulting data signal is provided to a computer connected to the modem.
Processing of the downstream signal in the tuner is as follows. A downstream signal input to CATV input terminal 11 is passed through a broad band amplifying circuit 1 and converted to a first intermediate frequency (IF) signal having the frequency of 950 MHz by a first local mixer circuit 2 and a first local oscillation circuit 7. Oscillation frequency of the first local oscillation circuit 7 is microcomputer-controlled by a PLL (Phase-Locked Loop) channel selection circuit 13. The IF signal converted to the first intermediate frequency is tuned by a first IF input tuning circuit 3 and amplified by a first IF amplifying circuit 4, channel selection is performed in a first IF output tuning circuit 5, and thereafter the signal is introduced to a second mixer circuit 6.
In the second mixer circuit 6, the first IF signal is converted to a second IF signal by a second local oscillation circuit 8, and the converted second IF signal is provided to an IF output terminal 12. The second local oscillator circuit 8 is under PLL control by PLL channel selection circuit 13 as is the first local oscillation circuit 7. Generally, 44 MHz is applied as the second intermediate frequency. The second IF signal output from a tuner output terminal is subjected to conversion to 5 MHz base line, A/D conversion, 64 QAM demodulation and MPEG processing, and thereafter, the resulting signal is provided as the data signal.
The tuner for a cable modem is always kept at a standby state for reception. Therefore, low power consumption is required. However, in the conventional double conversion type tuner for the cable modem, power consumption in the standby state is as much as 1 to 2 W which is considerably large as compared with power consumption during operation. Especially power consumption in the first mixer circuit 2 and first local oscillation circuit 7 is as high as about 70% of the entire power consumption.
In the conventional circuit described above, the double conversion method using two circuits, that is, first local oscillation circuit 7 and second local oscillation circuit 8, it becomes necessary to prevent interference between these circuits, and therefore it becomes necessary that the box design ensures strict electrical shielding. Further, the chassis must be designed such that there is sufficient space to further reduce interference. Accordingly, the outer shape becomes large and local spurious disturbance is more likely because of interference between the local oscillators, which results in higher possibility of communication error.
Further, in the conventional example, the input circuit for the IF amplifying circuit is not such a tuning circuit that tunes only to the desired signal but it is a broad band amplifying circuit 1. Therefore, when there is full wave input of CATV signals, distortion caused by radio interference is likely. It becomes difficult to increase power gain and to decrease noise figure, which means that there is higher possibility of communication error.
Further, since the first local oscillation frequency is in the band of 1 GHz to 2 GHz, it is difficult to improve microphonic noise and phase noise of the oscillator as a VCO (Voltage Controlled Oscillator). This results in communication error (deterioration of BERT) in the products.